There are various medical conditions which require a patient to be supplied with substantially pure oxygen. It is conventional practice to place a mask on the patient's mouth and nose and to supply pressurized oxygen to the mask through a reservoir bag and an appropriate valve assembly. A disadvantage of known masks of this kind is that the patient's demand for oxygen may be greater than the supply with the result that medical personnel or the patient have to tamper with the mask so that the patient also receives air from the external atmosphere.
It is an object of the present invention to provide a face mask which substantially overcomes the problem mentioned above.